Method and plant or increasing oil recovery by gas injection

ABSTRACT

A method and a plant for simultaneous production of a gas for injection into an oil field and production of methanol, dimethyl ether and/or other oxygenated hydrocarbons or production of higher hydrocarbons from natural gas is disclosed. An air separation unit (ATR) for production of pure nitrogen for injection and pure oxygen for production of synthesis gas (&#34;syngas&#34;) by authermal reformation of a natural gas is an essential part of the method and plant.

THE FIELD OF THE INVENTION

[0001] The present invention regards the use of natural gas in thedevelopment of industry and oil fields. In particular, the inventionregards a method and a plant for integrated production of synthesis gasand gas for injection into an oil reservoir.

THE BACKGROUND OF THE INVENTION

[0002] The reinjection of various gases into an oil reservoir in orderto enhance the oil recovery from the reservoir, and to stabilise it, haslong been known and used. Gases such as CO₂, N₂ and natural gas willreduce the surface tension between gas and oil, and thus contribute toboth increased recovery and stabilisation of the reservoir.

[0003] Natural gas as such may be injected into fields where the gasdoes not have a net value that exceeds the excess profits of increasingthe oil recovery in the field

[0004] Cleaning waste gas from the combustion on the productioninstallation can provide CO₂ for injection into oil reservoirs. Inaddition it has been suggested that CO₂ cleaned from the waste gas fromgas power plants be reinjected by laying a pipeline from a gas powerplant to the production installation for hydrocarbons.

[0005] N₂ may be produced together with O₂ in a so-called air separationunit (ASU). In an oil field, such an air separation unit will normallyproduce N₂ with a purity of >99.9% and oxygen-enriched air. There islittle or no need for this oxygen-enriched air on the oil field, and allor most of this is therefore released.

[0006] Separation of air into an “oxygen-depleted stream” and an“oxygen-enriched stream” is described in U.S. Pat. No. 5,388,645 andU.S. Pat. No. 6,119,778. The oxygen-depleted stream is used forinjection into a “solid carbonaceous formation” for improved recovery ofmethane and at least a part of the oxygen-enriched stream is used forreaction with a reactant stream containing at least one oxidizablereactant. Examples of processes are steel making operations, productionof non-ferrous metals, chemical oxidation processes and production ofsynthesis gas for Fischer-Tropsch synthesis of higher from natural gas.The oxygen-depleted stream has a nitrogen to oxygen volume ratio of 9:1to 99:1. A too high ration may lead to the formation of an explosivegas. An oxygen-depleted gas, e.g. nitrogen, for injection into an oilfield to enhance the production preferably includes less than 0.1%oxygen.

[0007] No other integration between the processes using theoxygen-depleted and oxygen-enriched streams is mentioned in U.S. Pat.No. 5,388,645 or U.S. Pat. No. 6,119,778.

[0008] Natural gas may also be used as feed for a number of processessuch as the production of methanol, dimethyl ether or other oxygenatedhydrocarbons, and/or synthetic fuel/propellant. This can take place inaccordance with known processes such as described in PCT/NO00/00404.

[0009] Plants for production of methanol and other oxygenatedhydrocarbons and/or synthetic fuel often require O₂ produced in an airseparation unit in order to produce synthesis gas (“syngas”). Syngas isa mixture of CO, CO₂, H₂ and water vapour and some non-reacted naturalgas. The syngas is used in various synthesis reactions, such as for theproduction of methanol and other oxygenated hydrocarbons, heavierhydrocarbons and ammonia. The oxygen produced in an air separation unitin such a plant is typically >95% pure oxygen, while the nitrogen willbe relatively impure nitrogen that is not suitable for otherapplications, and is therefore released to the atmosphere.

[0010] A process for preparation of higher hydrocarbons and forenhancing the production of crude oil from an underground formation isdescribed in CA 1,250,863. The off-gas from the synthesis plant isoxidised into mainly CO₂ and H₂O before it is injected into theunderground formation. Preferable the presence of nitrogen is avoided byusing oxygen from an air separation unit for all oxygen-demandingprocesses.

A SUMMARY OF THE INVENTION

[0011] According to the present invention, there is provided a methodfor increasing oil recovery from an oil reservoir in which method gas isinjected into the reservoir, comprising the steps of:

[0012] separation of air into an oxygen-rich fraction and anitrogen-rich fraction,

[0013] providing a natural gas stream and leading the natural gas streamand at least a part of the oxygen-rich fraction to a reformer forconversion to synthesis gas mainly comprising H₂, CO and CO₂ in additionto lower amounts of non-converted methane, water vapour and oxygen,

[0014] synthesis methanol or other oxygenated hydrocarbons or higherhydrocarbons from the synthesis gas in a synthesis unit,

[0015] withdrawing a waste gas from the synthesis unit

[0016] injecting the nitrogen-rich fraction and at least a part of thewaste gas into the oil reservoir to increase the oil recovery from thereservoir,

[0017] According to a preferred embodiment, the method further comprisesseparation of the waste gas from the synthesis unit into a CO₂-richfraction and a fraction low in CO₂ and using the CO₂-rich fraction forinjection into the oil reservoir.

[0018] Preferably the waste gas from the synthesis unit is combustedwith oxygen prior to separation into a CO₂-rich fraction and a fractionlow in CO₂.

[0019] The waste gas from the synthesis loop is preferably combusted atan elevated pressure, preferably at a pressure of from 2 to 100 bar,more preferably from 20 to 40 bar.

[0020] According to a second embodiment of the present invention, thewaste gas from the synthesis unit is separated into a CO₂-rich fractionand a fraction low in CO₂, and that the fraction low in CO₂ is thencombusted in a gas turbine or a furnace.

[0021] The fraction low in CO₂ that exits the synthesis loop may in apreferred embodiment be split into a hydrogen-rich fraction and afraction low in hydrogen, where the hydrogen-rich fraction is sent to aprocess that requires the addition of hydrogen, and the fraction low inhydrogen is combusted.

[0022] According to an embodiment it is preferred that the waste gasfrom the synthesis loop is combusted in a furnace or a turbine, and thatthe exhaust gas from the furnace or turbine is separated into a CO₂-richfraction that is injected into the oil reservoir, and a fraction low inCO₂.

[0023] Furthermore, it is preferred that the exhaust gas from thefurnace or turbine goes through secondary combustion in a catalyticsecondary combustion chamber before being separated into a CO₂-richfraction and a fraction low in CO₂.

[0024] It is also preferred that natural gas is added to the in thefurnace or turbine.

[0025] According to another embodiment it is preferred that a part ofthe synthesis gas is bypassed the synthesis unit.

[0026] Also provided is a plant for providing gas for downhole injectionfor pressure support in an oil reservoir for recovering of hydrocarbonsand production of methanol, dimethyl ether and/or other oxygenatedhydrocarbons or for production of higher hydrocarbons from natural gas,comprising:

[0027] an air separation unit for production of an oxygen-rich fractionfor supply to processes that require oxygen, and a nitrogen fraction forinjection;

[0028] a reformer for conversion of a mixture of natural gas, water andoxygen from the air separation unit into a synthesis gas comprisingmainly H₂, CO, CO₂ and small amounts of methane;

[0029] a synthesis unit for conversion of the synthesis gas forsynthesis of methanol or other oxygenated hydrocarbons, or for synthesisof synthetic fuel;

[0030] means for injecting gas into the reservoir;

[0031] means for transferring nitrogen from the air separation unit tothe means for injecting gas; and

[0032] means for transferring at least a part of a waste gas from thesynthesis unit to the means for injecting gas.

[0033] According to a preferred embodiment the means for transferringwaste gas from the synthesis unit comprises one or more separation unitsfor separating the waste gas into a CO₂-rich fraction that is led to theunit for injection for pressure support, and a fraction low in CO₂.

[0034] It is preferred that the plant further comprises a furnace or agas turbine for combustion of the waste gas from the synthesis unit anda line for leading oxygen for the combustion from the air separationunit to the furnace or gas turbine.

[0035] According to a preferred embodiment the plant further comprisesmeans of separating the waste gas from the synthesis unit into aCO₂-rich fraction and a fraction low in CO_(2,) and a gas turbine or afurnace for combustion of the fraction low in CO₂.

[0036] The plant preferably comprises means of splitting the low CO₂fraction of the waste gas from the synthesis unit into a hydrogen richfraction and a fraction low in hydrogen.

[0037] According to a preferred embodiment the plant further comprises afurnace or a gas turbine for combustion of the waste gas from thesynthesis unit and means of separating the exhaust gas from the furnaceor turbine into a CO₂-rich fraction that is led to the unit forinjection for pressure support, and a fraction low in CO₂.

[0038] It is preferred that the plant comprises a catalytic secondarycombustion chamber for secondary combustion of the exhaust gas from thefurnace or turbine prior to it being separated into a CO₂-rich fractionand a fraction low in CO₂.

[0039] Preferably, the plant further comprises a bypass line for leadingsome of the added natural gas past the reformer and the synthesis unit,to the furnace or turbine.

[0040] It is preferred that the plant further comprises a bypass linefor leading some of the synthesis gas past the synthesis unit.

[0041] By combining a plant for production of high-purity nitrogen withthe production of oxygen, the co-producing air separation unit onlybecomes 10-20% more expensive than an air separation unit that onlyproduces high-purity nitrogen for injection into oil fields. This allowssignificant cost savings, both for production of synthesis products suchas methanol and synthetic fuel, and for oil field injection.

A BRIEF DESCRIPTION OF THE FIGURES

[0042]FIG. 1 shows a schematic diagram of an embodiment of the presentinvention;

[0043]FIG. 2 shows a schematic diagram of alternative options for thepresent invention;

[0044]FIG. 3 shows an alternative embodiment of the present invention;and

[0045]FIG. 4 is an illustration of the economical impact of theintegrated process according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0046]FIG. 1 is a schematic diagram showing the principal features of apreferred embodiment of the present invention. Air is drawn in throughan air intake 1 to an air separation unit 2, where it is separated intothe main components nitrogen and oxygen. The air separation unit differsfrom traditional air separation units used for production of oxygen toreformers or for production of nitrogen for injection into an oil well,in that it produces nitrogen and oxygen with a high purity. The producednitrogen typically has a purity of >99.9%, while the oxygen typicallyhas a purity of 98-99.5%.

[0047] The nitrogen is passed through line 3 to a compressor 4 where itis compressed to the desired pressure, e.g. of the order of 50-400 bar.From the compressor 4, the compressed nitrogen stream is passed througha line 5 to a plant 6 for injection of gas into a field, a so-called EORunit (“Enhanced Oil Recovery”).

[0048] The oxygen is passed through a line 7 to a synthesis gasproduction unit, a so-called reformer 8.

[0049] Natural gas is fed to the plant through a gas inlet 9. Prior tothe natural gas being sent into line 11 to the reformer for productionof synthesis gas, it is treated in a pre-treatment unit 10 in whichsulphur compounds are removed in a conventional manner. The steam isthen saturated into the gas and/or added directly to the gas. Thesaturation may take place by means of a so-called saturator. Often, thegas is also treated in a so-called pre-reformer in order to convert allheavier hydrocarbons (C2+) before the gas is sent into the reformer 8.

[0050] In the reformer, the following are the main chemical reactions totake place during the production of synthesis gas:

CH₄+H₂O═CO+3H₂, steam reforming   1.

CH₄+3/2O₂═CO+2H₂O, partial oxidation   2.

CO+H₂O═CO₂+H₂, shift reaction   3.

[0051] Reaction 1 in the reforming reactor is highly endothermic, andthe heat required for the reaction may either be added through externalheating, such as in a steam reformer, or through a combination withinternal partial oxidation according to reaction 2, such as in anautothermal reformer.

[0052] In a steam reformer (SR), natural gas (NG) is converted in atubular reactor at a high temperature and relatively low pressure. Aconventional steam reformer consists of a large number of reactor tubesin a combustion chamber. Conventional steam reformers are operated in apressure range from approximately 15 to 40 bar. The outlet temperaturefor such a reformer can get up to 950° C. The heat required to drive thereaction is added by means of external heating in the combustion chamberin which the reformer tubes are installed.

[0053] The reformer may be top, bottom or terrace fired. The heat canalso be transferred to the reaction by means of convective heat as in aheat exchanger reactor. The ratio between steam and carbon in the feedgas is from 1.6 to 4. The composition of the synthesis gas may as anexample be expressed in stoichiometric numbers (SN═(H₂−CO₂)/(CO₂+CO)).The stoichiometric number for the product stream from the steam reformeris approximately 3 when the natural gas contains pure methane. A typicalsynthesis gas from a conventional steam reformer contains approximately3 volume % methane.

[0054] In an autothermal reformer (ATR), the synthesis gas productionmainly takes place through reactions 1 and 2, such that the heatrequired for reaction 1 is generated internally via reaction 2. In anATR, natural gas (methane) is led into a combustion chamber togetherwith an oxygen-containing gas such as air. The temperature of thecombustion chamber can get up to over 2000° C. After the combustion, thereactions are brought to an equilibrium across a catalyst before thegases leave the reformer at a temperature of approximately 1000° C. Thestoichiometric number, SN, for the product stream from an ATR isapproximately 1.6-1.8. The pressure may typically be around 30-40 bar,but a significantly higher pressure has also been proposed, such as inthe range 40-120 bar. The steam/carbon ratio may vary with the intendedapplication, from 0.2 to 2.5.

[0055] An alternative autothermal reformer makes use of a concept calledpartial oxidation (POX). Such a reformer does not contain any catalystfor accelerating the reactions, and will therefore generally have ahigher outlet temperature than an AMR

[0056] Reformation of natural gas may also take place through combinedreforming (CR), where the reformer section consists of a SR and an ATR.A combination of SR and ATR allows the composition exiting the reformersection to be adjusted by regulating the admission to the two reformers.SR will in CR be operated under somewhat milder conditions than in thecase of normal SR, i.e. at a somewhat lower temperature. This results ina slightly higher methane slippage in the outlet gas from the reformer.This methane content is converted in the subsequent ATR. The ratiobetween steam and carbon in the gas feed will, for such a reformer, liein the range 1.2 to 2.4, with a stoichiometric number, SN, of around 2or slightly on the high side of 2.

[0057] The desired composition of the synthesis gas will depend on theprocess for which it is to form the raw material. The optimumstoichiometric number for methanol synthesis s around 2.05, while thedesired stoichiometric number for production of synthetic fuel oftenlies in the range 1.6 to 1.9, as a higher stoichiometric number gives agreater yield of lighter hydrocarbons than that which is desirable.

[0058] After reforming, the synthesis gas is cooled by being heatexchanged with water to give steam. Upon further cooling, water from thesynthesis gas is condensed before being sent on via a line 12 to asynthesis unit 15.

[0059] The synthesis unit 15 may for instance be a synthesis unit forproduction of synthetic fuel (heavier hydrocarbons), comprising aso-called Fischer-Tropsch reactor (F-T reactor), or a synthesis unit forproduction of oxygenated hydrocarbons such as methanol and dimethylether.

[0060] When the synthesis unit 15 is a synthesis unit for production ofsynthetic fuel, the reaction may be described using the followingreaction equation:

nCO+2nH₂═[—CH₂—]_(n) +nH₂O

[0061] The reaction is highly exothermic. The Fischer-Tropsch synthesisis well known and is described e.g. in PCT/NO00/00404.

[0062] When the synthesis unit 15 is a synthesis unit for production ofmethanol, this synthesis takes place according to the following tworeaction equations:

CO+2H₂=CH₃OH

CO₂+3H₂═CH₃OH+H₂O

[0063] These exothermal reactions normally take place in a tubularreactor at a pressure of 60-100 bar and a temperature of 230-270 degreesC. The methanol synthesis is also well known and is described e.g. inPCT/NO00/00450.

[0064] Both of the above synthesis units comprise a number of componentsper se, and both processes normally include internal recycling ofnon-reacted synthesis gas in order to increase the carbon efficiency ofthe process.

[0065] The product from the synthesis unit 15 is extracted through aproduct outlet 16 for further treatment. Non-reacted synthesis gas andinert gas that collects in the loop can be removed from the synthesisunit 15 through line 17. This gas will in the following description bedenoted the waste gas from the synthesis unit. The amount andcomposition of the waste gas from the synthesis unit depends on thereleased methane in the synthesis gas from the reformer section, as wellas selected process parameters in the synthesis unit

[0066] For the methanol synthesis, the volume of waste gas from thesynthesis unit may be small. In this case, this gas may be released orcombusted prior to being released in order to avoid emissions ofhydrocarbons and CO.

[0067] If CO₂ is required for injection into the oil well in addition tonitrogen, or if environmental conditions require the emission of CO₂from the plant to be reduced, the waste gas from the synthesis unit mayalternatively be passed further to a CO shift converter 18 in whichnon-converted CO is converted according to the following reactionequation:

CO+H₂O→CO₂+H₂

[0068] in order to make it easier to separate out the carbon contents ofthe gas.

[0069] From the CO shift converter, the gas may if required be ledthrough a line 19 to a CO₂ recovery unit 20 in which CO₂ is separatedfrom the other constituents of the gas. CO₂ may be separated out bymeans of an absorption process, e.g. by means of an amine, a cryogenicprocess or possibly by means of membranes. From the recovery unit 20,CO₂ is led via a line 21, a compressor 28 and further via a line 29 toEOR unit 6.

[0070] The gas that was separated from CO₂ in the recovery unit 20, andwhich mainly consists of H₂, CH₄ and inert gases, is passed furtherthrough a line 22 to other uses in a unit 23.

[0071] The unit 23 may be a furnace in which the gas is combusted underthe addition of air, oxygen or oxygen-enriched air and provides heat fora heat-requiring process. Alternatively, the gas may be burnt in a gasturbine alone or as additional heating. Alternatively, hydrogen may beseparated from the gas before it is burnt or alternatively released.Hydrogen may here be used for hydrogen-requiring processes such as e.g.upgrading of oil by sweetening (removal of sulphur), for saturation ofunsaturated hydrocarbons and hydrocracking or for use in fuel cells.

[0072] If there is a great need for CO₂ for injection, the use of aso-called “once through” reactor in the synthesis unit 15 may also beenvisaged, i.e. a reactor without any recycling.

[0073]FIG. 2 shows alternative and optional embodiments of a plantaccording to the present invention. The figure is based on the sameprincipal units as FIG. 1, but some optional, and in some casespreferred, additional units besides bypass lines and feedback lines,have been added in order to ensure the highest possible conversion or inorder to adjust the composition of the gas.

[0074] A CO₂ recovery unit 13 may be interposed between the reformer 8and the synthesis unit 15. By so doing, a desired amount of CO₂ can beremoved from the synthesis gas and passed through a line 27 to thecompressor 28, where it is brought together with CO₂ from line 21. Thiscan be used as a means of changing the stoichiometric number of thesynthesis gas so as to give it an optimum composition.

[0075] When the synthesis unit 15 is a synthesis unit for production ofsynthetic fuel, synfuel, it may also be desirable to recycle non-reactedsynthesis gas from line 17 to the reformer via line 26. By recycling vialine 26, the H₂/CO ratio of the synthesis gas may be adjusted to thedesired value, i.e. around 2.0 or just below 2.0, and the CO yield andthereby also synthetic fuel yield may be increased by the high contentof CO₂ in the recycling gas suppressing further conversion of CO to CO₂through the shift reaction in the autothermal reformer. Here, it shouldbe noted that CO₂ is to be considered an inert gas in the F-T synthesis.

[0076] If the reformer produces more synthesis gas than can be convertedin the synthesis unit, some of the synthesis gas may be led from a line14 running between the CO₂ recovery unit 13 and the synthesis unit 15,and around the synthesis unit in a bypass line 25. This may also bedesirable if there is a wish to produce more heat or power in a furnaceor gas turbine 23.

[0077] In certain cases it may also be desirable to remove a volume ofnitrogen from line 5 out into a line 24 and bring this together with thegas in line 22, which is led to a turbine in unit 23 in order to controlthe combustion and generation of heat in this.

[0078] The units 13 and 20 for separating CO₂ from the remainder of thegas are known units. By the reformer 8 being supplied with pure oxygeninstead of air, the volume of gas to be treated becomes considerablysmaller. The separation in the units 13,20 may take place in a knownmanner by means of semi-permeable membranes or by absorption withsubsequent desorption, e.g. in a solution of alcohol amines.

[0079] The air separation unit 2 is preferably a plant based oncryogenic distillation, however it is also possible to use plants basedon pressure swing adsorption and/or membranes.

[0080]FIG. 3 shows a third embodiment in which non-converted synthesisgas from the synthesis unit 15 is combusted with pure oxygen in afurnace or gas turbine 30. Units having the same reference numbers as inFIGS. 1 or 2 indicate similar units with a similar functionality.

[0081] Oxygen is passed from line 7 through a line 40 and mixed with CO₂in a line 41, from where it passes into furnace or gas turbine 30. Thewaste gas from the furnace or gas turbine 30 goes via a line 31 to acatalytic secondary combustion chamber 32 in which the remaining fuel inthe form of CO, H₂ or non-combusted hydrocarbon is convertedcatalytically. The products of combustion from the secondary combustionchamber 32 are passed via a line 33 to a condensation unit 34, wherewater is condensed out and led out through a line 35, while CO₂ ispassed to the EOR unit 6 via a line 36.

[0082] CO₂ may be led from line 36 via a line 37 to a compressor 38. Forthis configuration, some compressed CO₂ must be recycled via line 41 tothe furnace or gas turbine 30 in order to maintain the combustiontemperature in this below a given maximum temperature.

[0083] If the requirement for heat and/or power is great, or there is arequirement for large volumes of CO₂, natural gas from line 11 may beled via a line 42 directly to the furnace or gas turbine 30.

[0084] Preferably, the combustion in the furnace or gas turbine 30 takesplace at an elevated pressure, such as from 2 to 100 bar, morepreferably from 20 to 40 bar. Having the combustion take place withpressurised oxygen facilitates the separation of CO₂ in the followingcondensation unit 34.

[0085] The great advantage of the present method and plant is that theyallow simple and energy efficient operation of the combined plant. Thepresent method also allows a more efficient and financially justifiablemethod of removing CO₂ from the waste gas from a methanol plant or plantfor production of synthetic fuel, for injection, so as to allow theemission of CO₂ to be eliminated or at least reduced considerably.

[0086] Those skilled in the art will appreciate that there may be unitsin the above figures for adjusting the pressure of the gases, such ascompressors or reducing valves that are not shown, but which arenecessary in order to match the pressures of the various units and toensure that the streams flow in the right direction. Moreover, there maybe units for heating or cooling, or heat exchangers that are not shownhere, the function of which is to optimise the energy efficiency of theplant.

EXAMPLE

[0087] Calculations have been carried out for a plant according to FIG.1 for production of methanol, which in addition comprises a bypass linethat leads some of the synthesis gas in line 12 past the synthesis unit15 and on to line 17.

[0088] The air separation unit can deliver 38 400 MTPD N₂ and 6400 MUDO₂. This air separation unit requires approximately 115 MW of power,which is delivered in the form of high pressure steam from the synthesisgas section.

[0089] The nitrogen is extracted at 3 bar and 0 degrees C. The gas iscompressed to 220 bar for reinjection. Compression requiresapproximately 304 MW.

[0090] The oxygen can be fed to an autothermal reformer for productionof synthesis gas from natural gas. The process operates with asteam/carbon ratio of 0.6. The temperature and pressure at the outletfrom the ATR is 1030 degrees Celsius and 45 bar respectively. See Table1 for the natural gas composition. Note! All compositions are given on adry basis, i.e. without water. TABLE 1 Composition of feeds to synthesisgas section Natural gas Oxygen Mole % Mole % CH₄ 83.7 C₂H₆ 5.2 C₃₊ 3.2CO₂ 5.2 N₂ + Ar 2.7 1.0 O₂ 0.0 99.0 H₂O 0.0 Sum 100 Total [Sm³/hr] 367000 190 850

[0091] Synthesis gas is compressed to 90 bar and mixed with recycledhydrogen in order to 20 achieve a stochiometric number of 2.56 prior tothe methanol synthesis. 10 000 MFPD of methanol is produced. TABLE 2 Gascompositions CO shift CO₂ ATR MeOH converted purified outlet reactorinlet Purge gas purge gas purge gas Mole % Mole % Mole % Mole % Mole %H₂ 62.9 65.9 27.3 38.7 52.6 CO 28.5 16.3 24.2 3.1 4.2 CO₂ 4.8 6.7 12.726.8 0.4 CH₄ 2.5 7.2 23.7 21.6 29.4 N₂ + Ar 1.3 3.9 12.1 9.8 13.4 Sum100 100 100 100 100 Total 1 093 000 3 488 000 113 000 136 000 100 000[Sm³/hr]

[0092] The waste gas from the synthesis unit, the purge gas, is sent toCO shift conversion. 35 t/h of steam is added in order to convert 85% ofCO to CO₂ in a low temperature shift converter (200 degrees Celsius).

[0093] 99% of the CO₂ in converted purge gas (equivalent to 1700 MTPDCO₂) is recovered in an MDEA process. Due to a high concentration of CO₂in the natural gas feed, this example includes CO₂ removal prior to ATR(equivalent to 800 MTPD CO₂), so that the total amount of recovered CO₂is 2500 MD. Recovered CO₂ is compressed to 220 bar, and may if sodesired be mixed with nitrogen prior to injection into the reservoir.CO₂ will then constitute around 6.2 weight % of the total injection gas.CO₂ constitutes a relatively small share of the total injectable gas.The cleaning of this may end up being so costly that it will only bedone if required by the authorities.

[0094] The remaining purge gas is used in fired heaters for superheatingof steam in power production and preheating of natural gas feeds. TABLE3 Power balance Power balance [MW] ASU incl. O₂ compression 115 CO₂recove 3 CO₂ compression 11 N₂ compression 304 Synthesis/methanolsection −155 Total 278

[0095] Here, the requirement for added power is approximately 280 MW.

[0096] Model for Evaluation of Economical Value

[0097] The benefit of using the nitrogen byproduct produced by theair-separation unit (ASU) of a GTL plant, for enhanced oil recovery(EOR), may be evaluated by analyzing the potential impact on the gasprice of the GTL plant. The natural gas price is without any doubt amajor factor determining the profitability of such a plant, and a creditwill be achieved for selling nitrogen.

[0098] Nitrogen and methane has roughly the same properties in EORoperations, essentially as pressure support. In the outset, we maytherefore assume that the value of the neat nitrogen is equivalent tothe gas price. We then will have:

[0099] P: Natural gas price in the area of the GTL facility.

P ^(Net)(GTL)=aP−bcP (Area gas price−credit for nitrogen sale)

[0100] where the coefficients are:

[0101] a) A factor reflecting the impact on the general gas price in thearea due to the integration. If P is the gas price with independent GTLand EOR operations, integration will significantly decrease the totaldemand for gas, and may therefore put pressure on the price, i.e., a <1.

[0102] b) The amount of nitrogen produced for a given amount (moles orenergy) of natural gas used by the GTL plant. For a facility with an ATR(autothermal reformer) unit, a typical oxygen consumption O2/NG is 0.63,giving N2/NG=2.34. This number will vary with the technical concept, gascomposition etc., but is used in the following to illustrate the impactof the EOR-GTL integration.

[0103] c) A factor presumably <1 taking into account that all thenitrogen produced may not be sold, e.g. due to overall well management,maintenance etc. Further, operational risks regarding continuousnitrogen delivery may put pressure on the nitrogen price.

[0104] The equation above may be modified further:

P ^(Net)(GTL)=aP−bcP+I+dS

[0105] where

[0106] I: The investment needed to implement the integration. This willessentially be some additional costs in the ASU to secure production ofnitrogen at a required purity, (additional) compression of the nitrogen,piping from the GTL to the EOR plant and possibly credit for energyintegration. All these factors are recalculated by accepted methods to acost (e.g. net present value) per amount natural gas used in the GTLplant.

[0107] S: Total savings (per amount natural gas) in the GTL gas price bythe integration. This means that

S=P−(aP−bcP+I)

[0108] d: The part of the savings that is passed onto the EOR operatorfor participating in the integration project, usually 0<d<0.5. Thefactor d might be a complicated function and there might also be overlapbetween the impact of factors c and d.

Illustrating Example

[0109] Assuming that a=1, b=2.34, c=1, I=0.2 (here 0.2 USD/MMbtu) andd=0.5, the impact of the integration is illustrated in FIG. 4. The linesare:

P ^(Net)(GTL)=aP=P (No EOR)   I:

P ^(Net)(GTL)=aP−bcP=−1.34P   II:

P ^(Net)(GTL)=aP−bcP+I=−1.34P+0.2   III:

P ^(Net)(GTL)=aP−bcP+I+dS=−0.17P+0.1   IV:

[0110] A few interesting things can be observed in the figure. First,line II indicates that there is a huge potential if a relevant EOR casecan be found. Line III shows that such an integration project will berobust against significant added investments. Further, line IVillustrates the point that even by passing half of the savings in thegas price over to the EOR operator, the net GTL gas price actually willbe lower for a high gas price in the area. At a nominal gas price of 1  IV: USD/MMbtu, the vertical arrows indicate that the added value forboth plants is 1.085   IV: USD/MMbtu of GTL feed gas.

[0111] There will be no incentive for a GTL/EOR integration at a nominalgas price below the crossing of lines I, III and IV, i.e. when I=bcP, orwhen the added investment equals the potential for nitrogen sales. Thisoccurs for a gas price of I/bc, or 0.085 USD/MMbtu in this example. Theonly case where a negative gas price will encourage integration is whenthe investment of integration is negative, a situation that may occurwhen there is no alternative use for the excess energy from the GTLplant.

1-19. (Cancelled)
 20. A method for increasing oil recovery from an oilreservoir in which method gas is injected into the reservoir,comprising: separating air into an oxygen-rich fraction and anitrogen-rich fraction; providing a natural gas stream and leading thenatural gas stream and at least a part of the oxygen-rich fraction to areformer for conversion to synthesis gas mainly comprising H₂, CO, andCO₂ in addition to lower amounts of non-converted methane, water vapor,and oxygen; synthesizing methanol or other oxygenated hydrocarbons orhigher hydrocarbons from the synthesis gas in a synthesis unit;withdrawing a waste gas from the synthesis unit; and injecting thenitrogen-rich fraction and at least a part of the waste gas into the oilreservoir to increase the oil recovery from the reservoir.
 21. Themethod according to claim 20, further comprising separating the wastegas from the synthesis unit into a CO₂-rich fraction and a fraction lowin CO₂ and using the CO₂-rich fraction for injection into the oilreservoir.
 22. The method according to claim 20, wherein the waste gasfrom the synthesis unit is combusted with oxygen prior to separationinto a CO₂-rich fraction and a fraction low in CO₂.
 23. The methodaccording to claim 22, wherein the waste gas is combusted at an elevatedpressure of from 2 to 100 bar.
 24. The method according to claim 23,wherein the waste gas is combusted at an elevated pressure of from 20 to40 bar.
 25. The method according to claim 20, wherein the waste gas fromthe synthesis unit is separated into a CO₂-rich fraction and a fractionlow in CO₂, and that the fraction low in CO₂ is then combusted in a gasturbine or a furnace.
 26. The method according to claim 21, wherein thefraction low in CO₂ is split into a hydrogen-rich fraction and afraction low in hydrogen, where the hydrogen-rich fraction is sent to aprocess that requires the addition of hydrogen, and the fraction low inhydrogen is combusted.
 27. The method according to claim 22, wherein thewaste gas is combusted in a furnace or a turbine, and that the exhaustgas from the furnace or turbine is separated into a CO₂-rich fractionthat is injected into the oil reservoir, and a fraction low in CO₂. 28.The method according to claim 27, wherein the exhaust gas from thefurnace or turbine goes through secondary combustion in a catalyticsecondary combustion chamber before being separated into a CO₂-richfraction and a fraction low in CO₂.
 29. The method according to claim27, wherein natural gas is added to the furnace or turbine.
 30. Themethod according to claim 20, wherein a part of the synthesis gasbypasses the synthesis unit.
 31. A plant for providing gas for downholeinjection for pressure support in an oil reservoir for recovery ofhydrocarbons and production of methanol, dimethyl ether and/or otheroxygenated hydrocarbons or for production of higher hydrocarbons fromnatural gas, comprising: an air separation unit for production of anoxygen-rich fraction for supply to processes that require oxygen, and anitrogen fraction for injection; a reformer for conversion of a mixtureof natural gas, water, and oxygen from the air separation unit into asynthesis gas comprising mainly H₂, CO, CO₂ and small amounts ofmethane; a synthesis unit for conversion of the synthesis gas forsynthesis of methanol or other oxygenated hydrocarbons, or for synthesisof synthetic fuel; means for injecting gas into the reservoir; means fortransferring nitrogen from the air separation unit to the means forinjecting gas; and means for transferring at least a part of a waste gasfrom the synthesis unit to the means for injecting gas.
 32. The plantaccording to claim 31, wherein the means for transferring waste gas fromthe synthesis unit comprises one or more separation units for separatingthe waste gas into a CO₂-rich fraction that is led to a unit forinjection for pressure support, and a fraction low in CO₂.
 33. The plantaccording to claim 31, further comprising a furnace or a gas turbine forcombustion of the waste gas from the synthesis unit and a line forleading oxygen for the combustion from the air separation unit to thefurnace or gas turbine.
 34. The plant according to claim 32, furthercomprising means for separating the waste gas from the synthesis unitinto a CO₂-rich fraction and a fraction low in CO₂, and a gas turbine ora furnace for combustion of the fraction low in CO₂.
 35. The plantaccording to claim 32, further comprising means of splitting the low CO₂fraction of the waste gas from the synthesis unit into a hydrogen richfraction and a fraction low in hydrogen.
 36. A plant according to claim33, further comprising means for separating the exhaust gas from thefurnace or turbine into a CO₂-rich fraction that is led to a unit forinjection for pressure support, and a fraction low in CO₂.
 37. The plantaccording to claim 36, further comprising a catalytic secondarycombustion chamber for secondary combustion of the exhaust gas from thefurnace or turbine prior to it being separated into a CO₂-rich fractionand a fraction low in CO₂.
 38. The plant according to claim 36, furthercomprising a bypass line for leading at least a portion of the addednatural gas past the reformer and the synthesis unit to the furnace orturbine.
 39. The plant according to claim 31, further comprising abypass line for leading at least a portion of the synthesis gas past thesynthesis unit.
 40. A plant for providing gas for downhole injection forpressure support in an oil reservoir for recovery of hydrocarbons andproduction of methanol, dimethyl ether and/or other oxygenatedhydrocarbons or for production of higher hydrocarbons from natural gas,comprising: an air separation unit configured to produce an oxygen-richfraction; a reformer configured to convert a mixture of natural gas andoxygen from the air separation unit into a synthesis gas comprisingmainly H₂, CO, CO₂ and lower amounts of methane; a synthesis unitconfigured to convert the synthesis gas for synthesis of methanol orother oxygenated hydrocarbons, or for synthesis of synthetic fuel; afirst line in communication with the synthesis unit and configured towithdraw a waste gas therefrom for transfer to a unit for injection; anda second line in communication with the air separation unit andconfigured to transfer nitrogen from the air separation unit to the unitfor injection.